ATLANTIC ATOC NETWORK DESIGN STUDIES

Principal Investigator: David R. Palmer
Collaborating scientist(s):
Geoff Brundrit, Department of Oceanography, University of Cape Town, South Africa
Leon Krige, Institute for Maritime Technology, Simon's Town, South Africa

Kurt Metzger, Department of Electrical Engineering and Computer Science, University of Michigan

Objective: The primary goal of the ATOC Program is to measure directly global ocean temperature trends using innovative underwater acoustic technologies, based on the fact that the speed of sound in water is proportional to temperature. The goal of this particular project is to conduct feasibility studies that will lead to an appropriate design for the Atlantic component of the planned ATOC global monitoring network.

Rationale: Since ocean temperature is a key indicator of global climate change, accurate temperature measurements, spanning the world's ocean basins and sampled over a long period, are essential to complement atmospheric measurements of global climate trends. The capability to make synoptic temperature estimates derived directly from acoustic travel time measurements was demonstrated in the 1991 Heard Island Feasibility Test (Refs. 1-3 below and sound file) where acoustic signals were transmitted from a location in the Southern Indian Ocean to 14 receiver stations, manned by nine international scientific teams, in the Atlantic, Indian, and Pacific oceans. The test demonstrated that acoustic signals of moderate intensity can be received over global paths with sufficient signal to noise ratios to measure propagation time and spatial variability.

Method: This effort focuses on the Atlantic Ocean because it is in the Atlantic that the interaction between the ocean and atmosphere is strongest and where one is likely to first observe in the ocean a long term climate trend. The approach involves design studies for an Atlantic network and sea trials to measure propagation characteristics between candidate nodes (source/receiver sites) in the network.

Accomplishment: Propagation studies were conducted between Cape Town and Ascension Island during November and December of 1992 (Refs. 5 and 6 below). The primary goal of the studies was to determine the extent that the acoustic path between Ascension and Cape Town is blocked by bathymetric features including the Walvis Ridge, which forms the southern boundary of the Angola Abyssal Plain, Valdivia Seamount, Bonaparte Seamount, and Grattan Seamount. These studies demonstrate that one can not only find appropriate source-receiver geometry's for which bathymetric interference can be minimized but also that the Cape Town site can be picked on the edge of the continental shelf -- a desirable location for logistical reasons. In addition, it was shown that for SOFAR propagation of incoherent sound over the distance of the experiment (4,300 km), a typical precision in travel time is 0.2 to 0.3 seconds. In only exceptional, high signal to noise situations can one expect to be able to measure arrival-time differences on nearby hydrophones to within 0.1 seconds. (Ref. 6 below) A feasibility study for a North Atlantic Acoustic Thermometry of Ocean Climate (ATOC) program has been competed (Ref. 7 below). The study was requested by the Scientific Committee on Oceanic Research (SCOR) Working Group 96 (Global Acoustic Monitoring of the Ocean).

Key reference:

1) D. R. Palmer et al., Reception at Ascension Island, South Atlantic, of the Transmission from the Heard Island Feasibility Test, NOAA Technical Memorandum ERL AOML-73 (February 1993) (available from the first author).

2) D. R. Palmer et al., Reception at Ascension of the Heard Island Feasibility Test transmission, Journal of the Acoustical Society of America vol. 96, 2432-2440 (1994).

3) D. R. Palmer, The Ascension Island Listening Station, Journal of the Acoustical Society of America vol. 90, 2330 (1991) (abstract); also Proceedings of the First International Meeting on Global Acoustic Monitoring of the Oceans, La Jolla, 1992.

4) D. R. Palmer, Tropical Atlantic Network Design Considerations, Proceedings of the Second International Meeting on Global Acoustic Monitoring of the Ocean, Brest, 1993.

5) G. Brundrit, L Krige, D. R. Palmer, J. Penrose, A. Forbes, and K. Metzger, Acoustic Thermometry of Ocean Climate-Feasibility Ascension Cape Town, Proceedings of the Second European Conference on Underwater Acoustics, Copenhagen, July 1994; also Proceedings of the Second International Meeting on Global Acoustic Monitoring of the Ocean, Brest, 1993.

6) David Palmer, Leon Krige, Geoff Brundrit, and Kurt Metzger, ATOC-FACT arrival-time differences, Journal of the Acoustical Society of America vol. 97, 3265 (1995) (abstract).

7) W. J. Gould, Y. Desaubies, B. M. Howe, D. R. Palmer, F. Schott, and C. Wunsch, Acoustic Thermometry in the Atlantic, Proceedings of the Second European Conference on Underwater Acoustics, Copenhagen, July 1994; also A Report to SCOR WG 96, Scientific Committee on Oceanic Research, International Council of Scientific Unions, 1994.

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